SD_BUS_MESSAGE_OPEN_CONTAINER - Online Linux Manual PageSection : 3
Updated :
Source : systemd 250
Note : sd_bus_message_open_container

NAMEsd_bus_message_open_container, sd_bus_message_close_container, sd_bus_message_enter_container, sd_bus_message_exit_container − Create and move between containers in D−Bus messages

SYNOPSIS#include <systemd/sd−bus​.h>int sd_bus_message_open_container(sd_bus_message *m, char type, const char *contents); int sd_bus_message_close_container(sd_bus_message *m); int sd_bus_message_enter_container(sd_bus_message *m, char type, const char *contents); int sd_bus_message_exit_container(sd_bus_message *m);

DESCRIPTIONsd_bus_message_open_container() appends a new container to the message m​. After opening a new container, it can be filled with content using sd_bus_message_append(3) and similar functions​. Containers behave like a stack​. To nest containers inside each other, call sd_bus_message_open_container() multiple times without calling sd_bus_message_close_container() in between​. Each container will be nested inside the previous container​. type represents the container type and should be one of "r", "a", "v" or "e" as described in sd_bus_message_append(3)​. Instead of literals, the corresponding constants SD_BUS_TYPE_STRUCT, SD_BUS_TYPE_ARRAY, SD_BUS_TYPE_VARIANT or SD_BUS_TYPE_DICT_ENTRY can also be used​. contents describes the type of the container's elements and should be a D−Bus type string following the rules described in sd_bus_message_append(3)​. sd_bus_message_close_container() closes the last container opened with sd_bus_message_open_container()​. On success, the write pointer of the message m is positioned after the closed container in its parent container or in m itself if there is no parent container​. sd_bus_message_enter_container() enters the next container of the message m for reading​. It behaves mostly the same as sd_bus_message_open_container()​. Entering a container allows reading its contents with sd_bus_message_read(3) and similar functions​. type and contents are the same as in sd_bus_message_open_container()​. sd_bus_message_exit_container() exits the scope of the last container entered with sd_bus_message_enter_container()​. It behaves mostly the same as sd_bus_message_close_container()​. Note that sd_bus_message_exit_container() may only be called after iterating through all members of the container, i​.e​. reading or skipping them​. Use sd_bus_message_skip(3) to skip over felds of a container in order to be able to exit the container with sd_bus_message_exit_container() without reading all members​.

RETURN VALUEOn success, these functions return a non−negative integer​. sd_bus_message_open_container() and sd_bus_message_close_container() return 0​. sd_bus_message_enter_container() returns 1 if it successfully opened a new container, and 0 if that was not possible because the end of the currently open container or message was reached​. sd_bus_message_exit_container() returns 1 on success​. On failure, all of these functions return a negative errno−style error code​.

ErrorsReturned errors may indicate the following problems: −EINVAL m or contents are NULL or type is invalid​. −EPERM The message m is already sealed​. −ESTALE The message m is in an invalid state​. −ENOMEM Memory allocation failed​. −EBUSY sd_bus_message_exit_container() was called but there are unread members left in the container​.

NOTESThese APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd ​pkg-config(1) file​.

EXAMPLESExample ​1. ​Append an array of strings to a message /* SPDX−License−Identifier: CC0−1​.0 */ #include <systemd/sd−bus​.h> int append_strings_to_message(sd_bus_message *m, const char *const *arr) { int r; r = sd_bus_message_open_container(m, 'a', "s"); if (r < 0) return r; for (const char *s = *arr; *s; s++) { r = sd_bus_message_append(m, "s", s); if (r < 0) return r; } return sd_bus_message_close_container(m); }Example ​2. ​Read an array of strings from a message /* SPDX−License−Identifier: CC0−1​.0 */ #include <stdio​.h> #include <systemd/sd−bus​.h> int read_strings_from_message(sd_bus_message *m) { int r; r = sd_bus_message_enter_container(m, 'a', "s"); if (r < 0) return r; for (;;) { const char *s; r = sd_bus_message_read(m, "s", &s); if (r < 0) return r; if (r == 0) break; printf("%s\n", s); } return sd_bus_message_exit_container(m); }

SEE ALSOsystemd(1), sd-bus(3), sd_bus_message_append(3), sd_bus_message_read(3), sd_bus_message_skip(3), The D−Bus specification[1]

NOTES 1. The D-Bus specification ­https://dbus.freedesktop.org/doc/dbus-specification.html
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